首页 > 其他分享> > Gstreamer基础教程9: 媒体信息采集

Gstreamer基础教程9: 媒体信息采集

2021-06-07 10:34:01 作者：互联网

文章目录

1.Goal
2.介绍
3.The GStreamer Discoverer
- 3.1 Compile
- 3.2 Code
4.解析
5.讨论

1.Goal

有时您可能想快速找出文件（或URI）包含哪种媒体，或者您是否能够播放所有媒体。您可以构建一个管道，将其设置为运行，并查看总线消息，但是GStreamer具有一个实用程序可以为您完成此任务。本教程显示：

如何恢复有关URI的信息
如何确定URI是否可播放

2.介绍

GstDiscoverer是在pbutils库（插件基础实用程序）中找到的实用程序对象，该对象接受URI或URI列表，并返回有关它们的信息。它可以在同步或异步模式下工作。
在同步模式下，只有一个要调用的函数gst_discoverer_discover_uri会阻塞，直到信息准备就绪为止。由于这种阻塞，通常对于基于GUI的应用程序来说不太有趣，并且使用了异步模式，如本教程中所述。
恢复的信息包括编解码器描述，流拓扑（流和子流的数量）和可用的元数据（如音频语言）。
例如，这是https://www.freedesktop.org/software/gstreamer-sdk/data/media/sintel_trailer-480p.webm的结果。
[外链图片转存失败,源站可能有防盗链机制,建议将图片保存下来直接上传(img-S5cXmR95-1623032260818)(:/b1d136ffd4f24504813101ca78ac4a45)]
以下代码尝试发现通过命令行提供的URI，并输出检索到的信息（如果未提供URI，则使用默认值）。
这是gst-discoverer-1.0工具的简化版本（基础教程10：GStreamer工具），该工具仅显示数据，不执行任何播放。

3.The GStreamer Discoverer

3.1 Compile

gcc basic-tutorial-9.c -o basic-tutorial-9 `pkg-config --cflags --libs gstreamer-1.0 gstreamer-pbutils-1.0`

3.2 Code

#include <string.h>
#include <gst/gst.h>
#include <gst/pbutils/pbutils.h>

/* Structure to contain all our information, so we can pass it around */
typedef struct _CustomData {
  GstDiscoverer *discoverer;
  GMainLoop *loop;
} CustomData;

/* Print a tag in a human-readable format (name: value) */
static void print_tag_foreach (const GstTagList *tags, const gchar *tag, gpointer user_data) {
  GValue val = { 0, };
  gchar *str;
  gint depth = GPOINTER_TO_INT (user_data);

  gst_tag_list_copy_value (&val, tags, tag);

  if (G_VALUE_HOLDS_STRING (&val))
    str = g_value_dup_string (&val);
  else
    str = gst_value_serialize (&val);

  g_print ("%*s%s: %s\n", 2 * depth, " ", gst_tag_get_nick (tag), str);
  g_free (str);

  g_value_unset (&val);
}

/* Print information regarding a stream */
static void print_stream_info (GstDiscovererStreamInfo *info, gint depth) {
  gchar *desc = NULL;
  GstCaps *caps;
  const GstTagList *tags;

  caps = gst_discoverer_stream_info_get_caps (info);

  if (caps) {
    if (gst_caps_is_fixed (caps))
      desc = gst_pb_utils_get_codec_description (caps);
    else
      desc = gst_caps_to_string (caps);
    gst_caps_unref (caps);
  }

  g_print ("%*s%s: %s\n", 2 * depth, " ", gst_discoverer_stream_info_get_stream_type_nick (info), (desc ? desc : ""));

  if (desc) {
    g_free (desc);
    desc = NULL;
  }

  tags = gst_discoverer_stream_info_get_tags (info);
  if (tags) {
    g_print ("%*sTags:\n", 2 * (depth + 1), " ");
    gst_tag_list_foreach (tags, print_tag_foreach, GINT_TO_POINTER (depth + 2));
  }
}

/* Print information regarding a stream and its substreams, if any */
static void print_topology (GstDiscovererStreamInfo *info, gint depth) {
  GstDiscovererStreamInfo *next;

  if (!info)
    return;

  print_stream_info (info, depth);

  next = gst_discoverer_stream_info_get_next (info);
  if (next) {
    print_topology (next, depth + 1);
    gst_discoverer_stream_info_unref (next);
  } else if (GST_IS_DISCOVERER_CONTAINER_INFO (info)) {
    GList *tmp, *streams;

    streams = gst_discoverer_container_info_get_streams (GST_DISCOVERER_CONTAINER_INFO (info));
    for (tmp = streams; tmp; tmp = tmp->next) {
      GstDiscovererStreamInfo *tmpinf = (GstDiscovererStreamInfo *) tmp->data;
      print_topology (tmpinf, depth + 1);
    }
    gst_discoverer_stream_info_list_free (streams);
  }
}

/* This function is called every time the discoverer has information regarding
 * one of the URIs we provided.*/
static void on_discovered_cb (GstDiscoverer *discoverer, GstDiscovererInfo *info, GError *err, CustomData *data) {
  GstDiscovererResult result;
  const gchar *uri;
  const GstTagList *tags;
  GstDiscovererStreamInfo *sinfo;

  uri = gst_discoverer_info_get_uri (info);
  result = gst_discoverer_info_get_result (info);
  switch (result) {
    case GST_DISCOVERER_URI_INVALID:
      g_print ("Invalid URI '%s'\n", uri);
      break;
    case GST_DISCOVERER_ERROR:
      g_print ("Discoverer error: %s\n", err->message);
      break;
    case GST_DISCOVERER_TIMEOUT:
      g_print ("Timeout\n");
      break;
    case GST_DISCOVERER_BUSY:
      g_print ("Busy\n");
      break;
    case GST_DISCOVERER_MISSING_PLUGINS:{
      const GstStructure *s;
      gchar *str;

      s = gst_discoverer_info_get_misc (info);
      str = gst_structure_to_string (s);

      g_print ("Missing plugins: %s\n", str);
      g_free (str);
      break;
    }
    case GST_DISCOVERER_OK:
      g_print ("Discovered '%s'\n", uri);
      break;
  }

  if (result != GST_DISCOVERER_OK) {
    g_printerr ("This URI cannot be played\n");
    return;
  }

  /* If we got no error, show the retrieved information */

  g_print ("\nDuration: %" GST_TIME_FORMAT "\n", GST_TIME_ARGS (gst_discoverer_info_get_duration (info)));

  tags = gst_discoverer_info_get_tags (info);
  if (tags) {
    g_print ("Tags:\n");
    gst_tag_list_foreach (tags, print_tag_foreach, GINT_TO_POINTER (1));
  }

  g_print ("Seekable: %s\n", (gst_discoverer_info_get_seekable (info) ? "yes" : "no"));

  g_print ("\n");

  sinfo = gst_discoverer_info_get_stream_info (info);
  if (!sinfo)
    return;

  g_print ("Stream information:\n");

  print_topology (sinfo, 1);

  gst_discoverer_stream_info_unref (sinfo);

  g_print ("\n");
}

/* This function is called when the discoverer has finished examining
 * all the URIs we provided.*/
static void on_finished_cb (GstDiscoverer *discoverer, CustomData *data) {
  g_print ("Finished discovering\n");

  g_main_loop_quit (data->loop);
}

int main (int argc, char **argv) {
  CustomData data;
  GError *err = NULL;
  gchar *uri = "https://www.freedesktop.org/software/gstreamer-sdk/data/media/sintel_trailer-480p.webm";

  /* if a URI was provided, use it instead of the default one */
  if (argc > 1) {
    uri = argv[1];
  }

  /* Initialize cumstom data structure */
  memset (&data, 0, sizeof (data));

  /* Initialize GStreamer */
  gst_init (&argc, &argv);

  g_print ("Discovering '%s'\n", uri);

  /* Instantiate the Discoverer */
  data.discoverer = gst_discoverer_new (5 * GST_SECOND, &err);
  if (!data.discoverer) {
    g_print ("Error creating discoverer instance: %s\n", err->message);
    g_clear_error (&err);
    return -1;
  }

  /* Connect to the interesting signals */
  g_signal_connect (data.discoverer, "discovered", G_CALLBACK (on_discovered_cb), &data);
  g_signal_connect (data.discoverer, "finished", G_CALLBACK (on_finished_cb), &data);

  /* Start the discoverer process (nothing to do yet) */
  gst_discoverer_start (data.discoverer);

  /* Add a request to process asynchronously the URI passed through the command line */
  if (!gst_discoverer_discover_uri_async (data.discoverer, uri)) {
    g_print ("Failed to start discovering URI '%s'\n", uri);
    g_object_unref (data.discoverer);
    return -1;
  }

  /* Create a GLib Main Loop and set it to run, so we can wait for the signals */
  data.loop = g_main_loop_new (NULL, FALSE);
  g_main_loop_run (data.loop);

  /* Stop the discoverer process */
  gst_discoverer_stop (data.discoverer);

  /* Free resources */
  g_object_unref (data.discoverer);
  g_main_loop_unref (data.loop);

  return 0;
}

4.解析

这些是使用GstDiscoverer的主要步骤

/* Instantiate the Discoverer */
data.discoverer = gst_discoverer_new (5 * GST_SECOND, &err);
if (!data.discoverer) {
  g_print ("Error creating discoverer instance: %s\n", err->message);
  g_clear_error (&err);
  return -1;
}

gst_discoverer_new创建一个新的Discoverer对象。第一个参数是每个文件的超时时间，以纳秒为单位（为简单起见，请使用GST_SECOND宏）。

/* Connect to the interesting signals */
g_signal_connect (data.discoverer, "discovered", G_CALLBACK (on_discovered_cb), &data);
g_signal_connect (data.discoverer, "finished", G_CALLBACK (on_finished_cb), &data);

链接感兴趣的signal。

/* Start the discoverer process (nothing to do yet) */
gst_discoverer_start (data.discoverer);

gst_discoverer_start启动发现过程，但我们尚未提供任何要发现的URI。接下来完成：

/* Add a request to process asynchronously the URI passed through the command line */
if (!gst_discoverer_discover_uri_async (data.discoverer, uri)) {
  g_print ("Failed to start discovering URI '%s'\n", uri);
  g_object_unref (data.discoverer);
  return -1;
}

gst_discoverer_discover_uri_async将提供的URI放入队列中进行发现。可以使用此功能将多个URI排队。当它们的发现过程完成时，将触发已注册的回调函数。

/* Create a GLib Main Loop and set it to run, so we can wait for the signals */
data.loop = g_main_loop_new (NULL, FALSE);
g_main_loop_run (data.loop);

通常的GLib主循环被实例化并执行。当从on_finished_cb回调调用g_main_loop_quit时，我们将退出循环。

/* Stop the discoverer process */
gst_discoverer_stop (data.discoverer);

discoverer完成操作后，请使用gst_discoverer_stop停止它，并使用g_object_unref取消引用它。
现在让我们回顾一下我们已经注册的回调on_discovered_cb：

/* This function is called every time the discoverer has information regarding
 * one of the URIs we provided.*/
static void on_discovered_cb (GstDiscoverer *discoverer, GstDiscovererInfo *info, GError *err, CustomData *data) {
  GstDiscovererResult result;
  const gchar *uri;
  const GstTagList *tags;
  GstDiscovererStreamInfo *sinfo;

  uri = gst_discoverer_info_get_uri (info);
  result = gst_discoverer_info_get_result (info);

在Discoverer已完成对一个URI的处理时该函数被调用，并为我们提供了包含所有信息的GstDiscovererInfo结构。
第一步是使用gst_discoverer_info_get_uri检索此调用引用的特定URI（有时会运行多个发现进程，在本示例中不是这种情况），并使用gst_discoverer_info_get_result()检索发现结果。

switch (result) {
  case GST_DISCOVERER_URI_INVALID:
    g_print ("Invalid URI '%s'\n", uri);
    break;
  case GST_DISCOVERER_ERROR:
    g_print ("Discoverer error: %s\n", err->message);
    break;
  case GST_DISCOVERER_TIMEOUT:
    g_print ("Timeout\n");
    break;
  case GST_DISCOVERER_BUSY:
    g_print ("Busy\n");
    break;
  case GST_DISCOVERER_MISSING_PLUGINS:{
    const GstStructure *s;
    gchar *str;

    s = gst_discoverer_info_get_misc (info);
    str = gst_structure_to_string (s);

    g_print ("Missing plugins: %s\n", str);
    g_free (str);
    break;
  }
  case GST_DISCOVERER_OK:
    g_print ("Discovered '%s'\n", uri);
    break;
}

if (result != GST_DISCOVERER_OK) {
  g_printerr ("This URI cannot be played\n");
  return;
}

如代码所示，GST_DISCOVERER_OK以外的任何结果都意味着存在某种问题，并且该URI无法播放。原因可能会有所不同，但是枚举值非常明确（GST_DISCOVERER_BUSY仅在同步模式下会发生，本示例中未使用）。
如果没有发生错误，则可以使用不同的gst_discoverer_info_get_*方法（例如，例如gst_discoverer_info_get_duration）从GstDiscovererInfo结构中检索信息。
由列表组成的信息位，例如tags和stream info，需要一些额外的解析：

tags = gst_discoverer_info_get_tags (info);
if (tags) {
  g_print ("Tags:\n");
  gst_tag_list_foreach (tags, print_tag_foreach, GINT_TO_POINTER (1));
}

tags是附加到媒体的元数据（标签）。可以使用gst_tag_list_foreach检查它们，该函数将为找到的每个tag调用print_tag_foreach（例如，也可以手动遍历列表，或者可以使用gst_tag_list_get_string搜索特定的tag）。print_tag_foreach的代码几乎是不言自明的。

sinfo = gst_discoverer_info_get_stream_info (info);
if (!sinfo)
  return;

g_print ("Stream information:\n");

print_topology (sinfo, 1);

gst_discoverer_stream_info_unref (sinfo);

gst_discoverer_info_get_stream_info返回一个GstDiscovererStreamInfo结构，该结构在print_topology函数中进行了解析，然后被gst_discoverer_stream_info_unref丢弃。

/* Print information regarding a stream and its substreams, if any */
static void print_topology (GstDiscovererStreamInfo *info, gint depth) {
  GstDiscovererStreamInfo *next;

  if (!info)
    return;

  print_stream_info (info, depth);

  next = gst_discoverer_stream_info_get_next (info);
  if (next) {
    print_topology (next, depth + 1);
    gst_discoverer_stream_info_unref (next);
  } else if (GST_IS_DISCOVERER_CONTAINER_INFO (info)) {
    GList *tmp, *streams;

    streams = gst_discoverer_container_info_get_streams (GST_DISCOVERER_CONTAINER_INFO (info));
    for (tmp = streams; tmp; tmp = tmp->next) {
      GstDiscovererStreamInfo *tmpinf = (GstDiscovererStreamInfo *) tmp->data;
      print_topology (tmpinf, depth + 1);
    }
    gst_discoverer_stream_info_list_free (streams);
  }
}

print_stream_info函数的代码也很不言自明：它也使用print_tag_foreach打印流的功能，然后打印相关的caps。
然后，print_topology寻找下一个要显示的元素。如果gst_discoverer_stream_info_get_next返回一个非NULL流信息，则它指向我们的后代，并且应该显示该信息。否则，如果我们是容器，则对通过gst_discoverer_container_info_get_streams获得的每个子代递归调用print_topology。否则，我们是最终的流，不需要递归（公认的Discoverer API的这一部分有些晦涩）。

Ref

gst_discoverer_new

GstDiscoverer *gst_discoverer_new(GstClockTime timout, GError **error))

创建具有timeout的GstDiscoverer。
timeout以纳秒为单位，范围为1s_{1h(GST_SECOND}3600*GST_SECOND)

gst_discoverer_start
gst_discoverer_discover_uri_async
gst_discoverer_info_get_uri
gst_discoverer_info_get_result
gst-discoverer_info_get_misc
gst_structure_to_string
gst_discoverer_info_get_*
gst_tag_list_foreach
gst_discoverer_stream_info_get_next
gst_discoverer_container_info_get_streams

5.讨论

本教程显示：

如何使用GstDiscoverer恢复有关URI的信息
如何通过查看使用gst_discoverer_info_get_result获得的返回码来确定URI是否可播放

标签：info,gst,discoverer,get,Gstreamer,采集,基础教程,print,data
来源： https://blog.csdn.net/qq_41592091/article/details/117651566